The present invention relates an ophthalmoscopy lens system, and in particular to an indirect ophthalmoscopy lens system that collects light exiting a patient's eye and creates a real, aerial image of the fundus and vitreoretinal structures outside of a patient's eye for use in diagnostic and surgical procedures of the central and peripheral regions of the retina. Indirect ophthalmoscopy lens systems generally fall into two categories, namely non-contact and contact. The non-contact indirect ophthalmoscopy lens is generally a single lens which is either handheld by the observer or mounted by a device in front of a patient's eye for collecting light emerging from the patient's eye and forming a real, aerial image that can be either observed directly or photographed. The contact indirect ophthalmoscopy lens includes two or more lens elements mounted in a fixed relationship in a frame or holder, wherein one of the lens elements is a contact lens element adapted to be placed on the cornea of a patient's eye and the other lens or lenses operate to converge light emerging from the patient's eye and passing through the contact lens element to form a real, aerial image. Both types of lenses are generally used in the context of diagnostic and surgical procedures with a slit lamp biomicroscope, operating microscope, or other binocular instrument for binocularly and stereoscopically viewing the real image formed outside of the eye by the indirect ophthalmoscopy lens, as well as with still cameras and video recorders for monitoring and recording the observed real image.
In the past, indirect ophthalmoscopy lens systems were conventionally designed to produce a planar, aerial image of the retina, or at least an image that had a flat appearance, and which was additionally free of aberrations including field curvature and astigmatism. Lens designers of indirect ophthalmoscopy lens systems have in the past utilized various aspheric optical surfaces, including aspheric conoids and aspheric non-conoids, to correct the retinal image so that it appears to be generally flat and in clear focus with a minimum of optical aberrations. Such lens systems often incorporate aspheric curvatures on at least one surface of a possible multiplicity of lens elements to achieve the optical correction.
U.S. Pat. No. 5,046,836 discloses an indirect contact ophthalmoscopy lens system in which both the contact lens element and an anterior lens element incorporate aspheric contours on each lens element surface, for a total of four aspheric surfaces. Single, non-contact indirect ophthalmoscopy lenses are also known in which one or both surfaces of the lens have aspheric designs as disclosed for example in U.S. Pat. No. 4,738,521, the object always being to achieve a generally planar image corrected for optical aberrations.
U.S. Pat. No. 5,007,729 discloses a three element indirect contact ophthalmoscopy lens system wherein the lens of the preferred embodiment is stated to produce an image that is actually slightly concave in the anterior direction. As explained in that patent, if the optics were to produce a truly planar image, an optical illusion would result in which the image would appear to be slightly convex to the observer, that is the central portion of the image would appear to be closer to the observer than the peripheral region of the image. In order to correct for this non-planar appearance, U.S. Pat. No. 5,007,729 teaches designing the optics so that the peripheral portions of the aerial image are slightly closer to the observer than the central portion of the aerial image so that the image, although in actuality being slightly concave, has the appearance of being relatively flat.
In contrast to the known indirect ophthalmoscopy lens systems and the associated planar, aerial image, there are direct ophthalmoscopy lenses such as the Hruby lens, Goldmann fundus lens and the various plano-concave style vitreoretinal surgery lenses, which produce a virtual image of the retina that is narrow in extent of field and appears highly curved to a degree that it confirms to the practitioner the actual curvature of the retina. Physicians find this apparent retinal curvature desirable in that it preserves a realistic sense of the actual shape of the fundus oculi.
Although the desirability of a visualized image curvature corresponding to the actual curvature of the retina obtained with the use of direct ophthalmoscopy lenses has long been appreciated, until now, this feature has not been achieved in indirect ophthalmoscopy to create an image of similar apparent curvature. One reason for this may be that to introduce a true curvature to the real image formed by an indirect ophthalmoscopy lens would cause those portions of the curved image not directly viewed to be out of focus to the observer. This would require the observer to continually readjust the focus of the slit lamp biomicroscope in order to maintain clear retinal imagery when viewing different portions of the aerial image. Even the slight curvature of the image as disclosed by U.S. Pat. No. 5,007,729, which is intended to provide the appearance of a relatively flat field, will require adjustment of the slit lamp microscope in order to maintain clarity and focus when viewing portions of the fundus image having different focal positions.
While flattening of the aerial image produced by an indirect ophthalmoscopy lens is necessary to maintain a clear and consistent focus over the entire fundus image, this flatness has its disadvantages. As mentioned above, the planar image does not present a realistic curvature of the visualized retina as compared to that of the virtual image produced by the direct ophthalmoscopy lens. Additionally, the same optical quality which imparts flatness to the image also causes instrumentation inserted into the eye, for example during vitreoretinal surgery, to attain a distorted curvature, potentially leading to misinterpretation of the location of the instrument and the region of the retina being viewed.